1. Field of the Invention
This invention relates in general to electrophotography and more specifically to a novel process for producing an electrophotographic imaging element, the resulting element, and the process of utilizing the element in electrophotography. In particular it is concerned with processes of making an electrophotographic imaging element by dissolving photoconductive charge generating monoazo dye, disazo dye, or derivatives of squaric acid in an organic primary amine or in a solvent mixture containing an organic primary amine and utilizing the solution so formed to solvent coat a photoconductive layer on a conductive substrate.
2. Description of the Prior Art
In the art of electrophotography, an electrophotographic imaging element containing a photoconductive layer is imaged by first uniformly electrostatically charging its surface and then exposing it to a pattern of activating actinic radiation, such as light. Radiation liberates holes and electrons from the photoconductor, causing it to be conductive where irradiated and thus selectively dissipates the charge in the irradiated areas of the photoconductor while leaving behind a charge which represents a latent electrostatic image in the non-irradiated areas. This latent electrostatic image is then developed to form a visible image by, for example, depositing finely divided electroscopic marking particles on the surface of the electrophotographic imaging element, which particles are attracted to the remaining areas of charge.
In practice the electrophotographic imaging element may be a homogeneous layer on a support or it may be a multi-layered structure including a layer of charge generating photoconductive material and layers of other materials. A substantial number of electrophotographic imaging elements including multiple layers are illustrated in the patent literature. For example, U.S. Pat. No. 3,041,166 discloses a layered structure consisting of an inorganic vitreous selenium photoconductor overlaid with an insulating photoconductive polymer. U.S. Pat. No. 3,165,405 discloses a structure designed for reflex imaging utilizing a two-layered zinc oxide-binder structure. U.S. Pat. No. 3,394,001 discloses an electrophotographic element including a conductive substrate carrying a photoconductive material, the photoconductive material being both underlaid and overlaid by an electron donor dye. U.S. Pat. No. 3,573,906 illustrates an electrophotographic element including photoconductive double layers containing an organic, possibly photoconductive insulating layer between the substrate and photoconductive vapor deposited selenium. In U.S. Pat. No. 3,598,582 is described a composite photosensitive device adapted for reflex exposure which employs a layer of organic photoconductive particles arrayed on a supporting substrate and overcoated by a layer of organic charge transport material. More recently many patents have issued which utilize a composite structure consisting of a conductive substrate, a charge generation layer and an organic charge transport layer as taught by U.S. Pat. No. 3,598,582. These include U.S. Pat. No. 3,713,820; 3,725,058; 3,824,099; 3,837,851; 3,839,034; 3,850,630 and 3,898,084. This last list of references can be divided into at least two categories, depending upon whether the charge generating photoconductive material is inorganic or organic. Where inorganic material is utilized it is found to be provided either in the form of particles in a binder matrix or in the form of a continuous film produced, for example, by vapor or vacuum deposition. No examples are known of the dissolution of inorganic materials in a solvent and the coating of the resulting solution onto a support to form a charge generating photoconductive layer. Similarly, with rare exceptions, where the charge generating photoconductive material is an organic compound it is normally dispersed in the form of pigment particles in a matrix binder which are coated in particulate form on a substrate. The known exception is in those instances where the organic photoconductor is itself a thermoplastic polymer capable of dissolution in a wide range of standard hydrocarbon or halogenated solvents, such as in the case of polyvinyl carbazole compounds, and the like. In no instance is the dissolution of an organic coloring material to form a photoconductor known. In the vast majority of organic charge generating material containing electrophotographic elements described in the literature the organic photoconductive material is provided as a layer consisting of discrete particles dispersed in a binder matrix. This is specifically the case as to electrophotographic elements reported in the literature as using coloring materials such as monoazo, disazo, and squaric acid derivative materials.
The utilization of monoazo, disazo, and squaric acid derivative materials in electrophotographic elements appears to be somewhat limited in the patent literature. One known example is U.S. Pat. No. 3,775,105 which describes a technique for enhancing the sensitivity of a photoconductor by the inclusion of milled submicron disazo particles. While the disazo compounds in this patent are milled in a solvent they are not reported as being dissolved, but rather remain as particles. One specific disazo compound reported for use in this manner is chlorodiane blue. U.S. Pat. No. 3,824,099, also noted above as a multi-layered structure, discloses an electrophotographic element including a conductive substrate, a layer of ground squaric acid methine particles in a matrix binder coated onto the substrate, and a charge transport layer of tri-aryl pyrazoline. Again, while the binder dispersion is prepared in a solvent, the squaric acid methine particles are not reported as being dissolved despite the fact that they are referred to as dyes in the patent. Indeed, but for the binder , it appears that the charge generating photoconductive material of this patent could not be adhered to the substrate.
U.S. Pat. No. 3,837,851 teaches another multilayered electrophotographic element including a conductive substrate, a photoconductive charge generating layer, and a charge transport layer of tri-aryl pyrazoline. While most of its charge generating material examples are directed to vapor deposited inorganic compounds, this reference also teaches the use of organic charge generating layers including disazo compounds, phthalocyanine compounds, and squaric acid derivatives. In no instance is the dissolution and solution coating of such organic charge generating materials disclosed by this patent. Finally, U.S. Pat. No. 3,898,084 discloses an electrophotographic element utilizing very small particles of disazo pigment dispersed in a matrix binder as a charge generating layer. Typically the charge generating layer is in the form of photoconductive particles in a binder coated on a conductive substrate. The charge generating layer may be overcoated with a charge transport layer. While many solvents are disclosed for utilization in the milling and coating of the disazo compounds of this patent they are not reported as dissolving the disazo compounds. Therefore, the particles require a binder matrix to fix them to the substrate.
It is thus seen, that while examples of disazo dye, and derivatives of squaric acid have been utilized in electrophotographic elements, no reference is known which discloses how these materials may be dissolved and used to form an electrophotographic imaging element by solution coating. Similarly, it is not suggested in the prior art that, once dissolved, coloring materials of this kind can be coated on a support without the use of a binder matrix.
Other solvents for these monoazo and disazo materials are known or have been earlier discovered and used, for example, for dye recrystallization. See, for example, H. E. Hunziker and R. B. Larrabee, IBM Technical Disclosure Bulletin, Vol. 18, No. 3, August 1975, p. 908. These include sulfuric acid, liquid ammonia and nitrobenzene. However, these materials are so corrosive, or difficult to handle, or provide so little dissolution that their use to dissolve and solution coat photoconductive charge generating coloring materials is not practical. Additionally, because of the corrosive nature of some of these solvents they are difficult to handle and could substantially destroy both the coating system or the substrate upon which the materials are to be coated.
Examples of composite photoconductive structures including polymer or charge transport materials in the same layer as the charge generating material include U.S. Pat. Nos. 3,121,006 and 3,121,007.
It is now considered to be desirable to be able to solution coat charge generating photoconductive materials. Solution coating avoids the requirement of grinding and milling the photoconductive pigment and avoids the use of additional expensive binder material and large volumes of solvent for the binder in order to secure the pigments to the substrate. Other shortcomings in grinding and milling operations are noted below. Normally, grinding or milling requires a great deal of time and can only be carried on one batch at a time. Therefore, in order to prepare a photoconductive layer including ground particles in a binder matrix it is required that batches be prepared discontinuously, or that large capital investments in duplicate grinding equipment be made in order to provide continuous batches of coating material.
Additionally, experience with disazo particles in a binder matrix coated on a substrate and dried have normally resulted in relatively thick coatings having limited photosensitivity. In order to improve photosensitivity the additional step of buffing the surface of the binder-disazo coating has been required. This additional step is not only expensive and time consuming, but it is found to affect the reproducibility of photosensitivity from imaging element to imaging element, and to even affect it at different portions of the same element. By contrast solution coating, with or without a binder, allows fast, continuous dye solution preparation and coating and eliminates the need for batch operations. Such continuous operation provides simpler and more economical operation with greater reproducibility of coating concentration and thickness, and thus also with greater reproducibility of photosensitivity. Additionally, solution coating affords ease of scale up and the preparation of extremely thin coatings, which require no further treatment, such as buffing, after they have been applied to a substrate.